Field of the Invention
The invention relates to a fuel cell with at least one active chamber and an oxide solid electrolyte. The active chamber is formed by a pair of electrically conductive plates which are disposed one above the other, are divided by an ion-conducting active layer structure into sub-chambers lying one above the other and being closed off from one another, and which are surrounded by an outer assembly region. The active chamber is closed off laterally in gas-tight fashion by insulating segments, the insulating segments being arranged in the outer assembly region and keeping the plates spaced apart from one another. Further, the surface of the plates is profiled in the region of the active chamber and forms contact segments each of which one electrode face of the ion-conducting active layer structure rests, and gas conduits, through which a reactive gas can be fed through each of the two sub-chambers. Such a fuel cell is described in German patent disclosure DE-A-42 37 602 (PCT/DE93/01017), the disclosure of which is herein incorporated by reference. The invention pertains in particular to a high-temperature fuel cell having a solid electrolyte of ion-conducting oxide (so-called solid oxide fuel cell, SOFC), and also to a method for coating the metallically conductive plates used in a cell of that type, and to a method for producing the cell.
Many prior art fuel cells are tubular in shape. However, fuel cell with a planar layer structure reach an energy density which is substantially higher (according to current experience, about 1 MW/m3). The energy generation is in this case based on the controlled chemical conversion of oxygen ions and hydrogen into water, i.e., on the so-called cold hydrogen-oxygen reaction, which respectively takes place in an active chamber which is divided by an active layer structure into two sub-chambers lying above one another. A hydrogen-containing gas (for example hydrogen), a gas mixture (H.sub.2 /CO/CO.sub.2) produced from conventional hydrocarbon fuels, or a mixture (H.sub.2 /CO/CH.sub.4) produced by reforming natural gas, flows through one of the sub-chambers, while an oxygen-containing gas, for example oxygen or air, flows through the other sub-chamber.
The invention herein starts out from a high-temperature fuel cell as described in DE-A-42 37 602. The most important constituent of the active layer structure is a layer of an electrolyte. The electrolyte surface respectively adjoining the sub-chambers lying above and below is designed as an electrode whose electrode potentials are tapped via contact segments of electrically conductive plates which lie opposite the electrolyte surfaces. By series connection of a plurality of active chambers of this type, bounded above and below by electrically conductive plates, the potential differences across the individual electrolyte layers can be added to form considerable voltages.
The following description of the prior art fuel cell refers to FIG. 1, in which there is shown the basic structure of a sandwich of this type, covered by a metallically conductive base plate 1 and cover plate 2. The prior art fuel cell has the following features:
each pair of metallically conductive plates 3, 4, which are disposed one above the other, form therebetween an active chamber K surrounded by an outer assembly region A. The active chamber is subdivided by an ion-conducting active layer structure (for example a plate 14 made of the above-mentioned solid electrolyte, and one electrode layer 12, 13 on each side of the electrolyte plate) into two sub-chambers 11, 11'. The subchambers 11, 11' lie one above the other and are closed off from one another; PA1 the active chamber K is closed off laterally in a gas-tight fashion by insulating segments 5. The insulating segments 5 are disposed in the outer assembly region A and they maintain a distance d between the plates 3, 4; and PA1 the surfaces 6, 7 of the plates 3, 4, respectively, are profiled in the region of the active chamber K and they form contact segments 8, 9, which are abutted by the active layer structure with are respective electrode face 12, 13. PA1 a pair of electrically conductive plates having surfaces facing toward one another and defining an active chamber therebetween; an ion-conducting active layer structure disposed between the plates and dividing the active chamber into sub-chambers lying one above the other and being closed off from one another; and an outer assembly region surrounding the active chamber; PA1 a plurality of insulating segments disposed in the outer assembly region, the insulating segments laterally closing off the active chamber in gas-tight fashion and maintaining the plates at a given spacing distance from one another, the insulating segments comprising an impermeable coating firmly adhering to the surface of at least one of the plates in the outer assembly region, the impermeable coating insulating against electron conduction, and an impermeable filler filling a remaining gap between the surfaces of the plates; PA1 the surfaces of the plates being profiled in a region of the active chamber and forming a contact segment contacting a respective electrode face of the ion-conducting active layer structure, and the profiled surfaces having gas conduits formed therein for feeding reactive gas through the sub-chambers. PA1 producing a plurality of profiled plates to be stacked one above the other at a given spacing distance and at least partly coating the plates; PA1 producing a plurality of active layer structures for active chambers of the fuel cell; PA1 forming foils of an amorphous oxide powder and a binder, the foils having a thickness greater than the given spacing distance in a finished fuel cell between the at least partly coated plates, having a cross-sectional area substantially corresponding to a cross-sectional area of the fuel cell, and having voids formed therein for receiving the the active layer structures; PA1 stacking the plates, the active layer structure and the foils into a sandwich structure defining a plurality of vertically stacked active chambers, wherein each active layer structure is placed in a void of a respective foil between two plates; and PA1 sintering the sandwich structure until a height thereof reaches a given height of the fuel cell.
In the case of an impermeable solid, the total electrical conductivity .lambda..sub.total can be divided into a "metallic" conductivity .lambda..sub.el, which depends on the electrons in the conduction band of the solid, whose high mobility decreases with increasing temperature, and an ion conductivity .lambda..sub.ion, the basis of which is the restricted mobility of ions in the solid (in the case of an oxidic solid electrolyte: O.sup.2-) and increases with increasing temperature: EQU .lambda..sub.total =.lambda..sub.el +.lambda..sub.ion
The partial conductivities .lambda..sub.el and .lambda..sub.ion are described by the concentration of the corresponding charge carriers and by "transport numbers" t.sub.el and t.sub.ion which depend on the value of the charges and their mobility within the crystal structure of the solid.
For fuel cells, it is important for the oxygen and the hydrogen supplied to the separate sub-chambers to be ionized by electron exchange with the correspondingly charged electrodes of the chambers. During the formation of H.sup.+, electrons are donated to the electrode of the corresponding sub-chamber and are discharged via the corresponding plate before--on the other side of the plate--either being given up to form O.sup.2- or being tapped as a current in a load circuit connected to the cell. The electrolyte layer thereby allows the ions to combine and form H.sub.2 O through ion migration.
It is therefore necessary, with a view to construction, for the plates between individual active chambers to seal these chambers against diffusion of the reaction gases and the ions. In physical terms, at least the plates 3, 4 which lie between the base plate 1 and the cover plate 2 and respectively separate two neighboring chambers, must have a high electron conductivity in order to permit the formation of H.sup.+ in the sub-chamber of one of the chambers, and simultaneously the formation of O.sup.2- in the sub-chamber of the other chamber. They are therefore referred to as bipolar plates (BIP). Exactly the opposite is true for the material of the electrolyte layer: it must have a low electron conductivity, so that the ionization potential is sustained at the electrodes, but must permit the requisite migration of the ions. Thus, as regards the ratio of the transport numbers for electrons and ions, which characterizes the mobility of the electrons and ions at the operating temperature of the fuel cell (600.degree. C. to 1000.degree. C.), then this ratio must be set greatly in favor of electrons for the BIP and greatly in favor of the ions (in particular oxygen ions) for the electrolyte layer.
Zirconium oxide (i.e., zirconium dioxide ZrO.sub.2) is generally the material of choice for the electrolyte 14. The linear coefficient of thermal expansion of the electrolyte 14 must be compatible with the linear expansion of the BIPs 3, 4, in order to ensure that the overall assembly is stable and leakproof. The active layer structure K is thereby divided into a plurality of mutually adjacent units. The edge of the layer structure K, or of its units, is held in an inner assembly region B in such a way that the sub-chamber 12 consists of a plurality of mutually adjacent spaces which are each closed off gas-tightly from the sub-chamber 11.
The plates themselves may consist of an electrically conductive ceramic which is particularly tailored to the requirements of this field of application, of steel with high shape stability or of an alloy in which, for example, an oxide is dispersed. A particularly suitable example of an oxide dispersion alloy (ODS alloy) of this type is a chromium-based alloy containing 5% iron and 1% yttrium oxide (Y.sub.2 O.sub.3), the chromium-iron alloy being essentially matched to the linear expansion of the electrolyte, while the oxide dispersed therein serves primarily to improve the corrosion properties of the alloy.
FIG. 1 also shows a feed port 15 for a gas which is fed into the gas conduits 10 that are perpendicular to the plane of the drawing. The gas is discharged via a non-illustrated discharge port. Correspondingly, the arrows 16, 16', 16" also indicate that the other gas is fed via the gas connection 17 into the channels between the contact segments 8, and thereby through the other sub-chambers of the active chambers, before being discharged through a discharge connector (which is not illustrated in FIG. 1) on the other side of the fuel cell. The water produced during the cold hydrogen-oxygen reaction is discharged from the fuel cell through these gas flows, together with the residual enthalpy of reaction which is not converted into electrical energy.
If the bipolar plates 3, 4 are not sufficiently insulated from one another by the segments 5, then internal electrical losses occur which can greatly impair the efficiency of the fuel cell. In order for internal electrical losses of this type not to exceed 1 per thousand (.sup.o /oo), sufficient insulation must be provided in the segments 5 of the outer assembly region A.
In addition, the fuel cell should have substantially no leaks through which one of the reaction partners can escape. It is customary to seal the fuel cell in the assembly region using a solder glass which is stable at high temperatures, but for a customary layer thickness of about 700 .mu.m, the leaktightness of such a wide soldered gap raises considerable difficulties.
In particular, it raises technical difficulties of filling such wide soldering sites with solder material, without giving rise to internal stresses or even micro-cracks which could lead to a failure of the soldering sites, an aggravating factor being that sealing the soldering site requires a sintering process which is usually associated with a reduction in volume.
It is also difficult for solder glass, which fills a wide gap, to be sintered in such a way that the soldering material does not start to flow and gain access to regions of the active chamber in which it has a disruptive effect. A solder glass is generally an oxide powder (usually white) which is mixed with a binder (for example an organic binder), in order to permit controlled application of the soldering material. The soldering process itself is performed by heating, during which the organic binder escapes and the remaining oxide is fused or sintered and thereby forms a gas-impermeable amorphous filler.
The sintered amorphous filler has electron conductivity which, although it decreases with increasing temperature, cannot be ignored, especially if chromium oxide diffuses from the bipolar plate into the solder glass during the fusion or sintering process, for example. During the sintering, it is also possible for chromium oxide that has diffused in to be reduced and for chromium boride or other components which impair the insulation to be formed.
Vapors which are produced when the binder is burnt off or escapes may also be toxic and difficult to dispose of. For this reason, the amount of solder glass should be limited. The escaping binder can also damage other surfaces in the active chamber, in particular the sensitive electrode surfaces of the active layer structure.
It has therefore been proposed heretofore, instead of completely filling the gap width d with solder glass of this type, to solder a corresponding frame made of an electrically insulating ceramic (for example a spinel, MgAl.sub.2 O.sub.4) into the soldering gap. Correspondingly, the assembly region according to the prior art thus comprises the layer sequence: BIP/solder glass/spinel/solder glass/BIP.
An oxide frame of this type, the height of which should be only a few 100 .mu.m, nevertheless requires a large outlay on production and careful treatment. It can therefore only be used in the laboratory, and not under economic manufacturing conditions.
At the high operating temperatures, the surfaces of the BIPs forming the gas conduits on the cathode side of the electrolyte layer are also particularly sensitive. Indeed, oxygen corrosion may occur there in the alloy of the BIPs, in particular the formation of chromium oxide. For its part, the chromium oxide may, through solid-state diffusion, reach other components of the SOFC and damage them. Similarly, hydrogen corrosion or carbon corrosion may occur on the surface of the BIP on the anode side in the gas conduits, which may in the long term embrittle and destroy the corresponding contact segments.